We now focus on the off-diagonal term \(M_{JQ}\)  of the transport matrix.
This coefficient expresses the dependence of the excess solute flux on the bulk pressure gradient.
A pressure gradient exerts a force on a volume of fluid rather than on individual particles.
As a first approximation, one might tend to connect the thermodynamic force acting on the system to the microscopic force as (see e.g [1]}, [2]}, [3]}, [4]}, [5]}):
\(\mathbf {F}^{P} = -P/ c^{\Omega }\;.\) 
